The present invention relates to an electronic device with double-wire bonding, a manufacturing method thereof, and a method for checking the intactness of the bonding wires of this electronic device.
As is known, in electronic power devices, the connection between one or more of the contact pads of the die in which the device is provided and the respective pins of the lead frame to which the die is secured (typically by a bonding layer), is frequently obtained by double-wire bonding formed by two bonding wires that are typically made of gold and are arranged in parallel with one another.
The double-wire bonding is particularly useful in cases in which it is important to guarantee bonding having low resistivity during the conduction phase or a high current conduction capacity or both. In fact, for example, a single 2 mil gold wire cannot conduct a current greater than 2 A for an indefinite time, whereas if a second wire is added in parallel to the first, the current conduction capacity is in fact doubled.
In addition, in some applications, in order to reduce further the resistivity of the bonding during the conduction phase, and to increase further the current conduction capacity, use is also made of two pins of the lead frame, which have the same function, and are thus connected to one another outside the electronic device, and each have double-wire bonding to the respective pads.
In double-wire bonding, it is therefore necessary to ensure in a final test phase, that there is intactness and satisfactory bonding of all the bonding wires, by an automatic checking process, since, with the present technology, the bonding process has a defectiveness of approximately 50-100 ppm, and the intactness of the bonding wires is not guaranteed by the assembly operation, and the lack of one or more bonding wires could make the electronic power device unreliable.
The conventional measuring methods, such as pin to pin continuity measurements, require execution of operations that are somewhat complex, and particularly demanding in terms of testing time, in order to distinguish faulty bonding, in which a single bonding wire is present, from satisfactory bonding, in which both bonding wires are present, since the resistance of the gold wires is negligible compared with the overall resistance of the circuit being measured.
For example, a 2 mil gold wire 3 mm long has a resistance of 33 mxcexa9, and on the assumption that a measurement is carried with a current of 1 A, along the bonding wire there is consequently a voltage drop of 33 mV, which is approximately 100 times smaller than the voltage drop (approximately 3 V) on the diode (and on its internal resistor) which is typically arranged in series with the bonding wire.
Therefore, taking into account the dispersion of the values of the voltage drop on the diode, the voltage drop caused by the presence of a single bonding wire cannot be distinguished from that caused by the presence of two bonding wires.
A known solution used in order to overcome this problem consists of doubling the pad to which one end of the bonding wires is connected and the bonding wire, thus providing two separate pads which are connected to the same pin by different bonding wires.
This solution is shown schematically in FIGS. 1 and 2. In particular, FIG. 1 shows schematically an electronic device 1 comprising an output transistor, in the example an MOS transistor, one terminal of which is connected to a pad 3, which in turn is connected to a pin 4 via double-wire bonding indicated as 5a and 5b. 
FIG. 2 shows schematically the same electronic device, in this case indicated as 8, as modified on the basis of the above-described known solution. In particular, in FIG. 2, the output transistor 2 is doubled into the output transistors 2a and 2b, the output terminals of which 6a, 6b are each connected to a corresponding pad 3a, 3b, into which the pad 3 has been doubled. The output transistors 2a and 2b have control terminals 7a, 7b, which are separated so that their operation can be controlled independently; the pin 4 is connected by the bonding wire 5a to the pad 3a, and by the bonding wire 5b to the pad 3b. 
By means of this solution, by switching on separately the output transistor 2a or 2b, it is possible to check the correct bonding of the respective bonding wire 5a or 5b. 
Although this solution makes it possible to check separately each bonding wire easily, by switching on separately each electronic component which is connected to the latter, it has the disadvantage that it requires an increase in the surface area occupied by the electronic device, due to the doubling of the electronic components that are connected at the output, and of the corresponding pads, as well as to the need to isolate electrically these doubled electronic components, and to the presence of additional electronic components for execution of the checking functions, which is highly undesirable in view of the continual trend towards ever-increasing miniaturization of integrated devices.
This disadvantage is all the more serious in cases in which, instead of having a single output electronic component to be doubled, there is a bridge configuration, in which all four of the electronic components of the bridge must be doubled.
This solution has the further disadvantage in that it involves a complication in the driving stage (not shown) of the electronic power device, which must be able to switch on separately each electronic component doubled, and that it requires introduction of a further command in order to activate the checking function and to select the final stage to be activated.
A solution proposed in order to eliminate the above-described disadvantages is described in European Patent EP-0622733 in the name of the applicant.
This solution does not require doubling of pads, and is based on the fact that, when a somewhat high current is passed through double-wire bonding, this gives rise to substantial dissipation of power by the bonding wires, and thus to heating of the latter, such that, since the resistance of gold wires is strongly dependent on the temperature, by monitoring the development over a period of time of the voltage drop caused by the current, it is possible to distinguish the double-wire bonding from that which is defective, in which only one of the bonding wires has been bonded correctly.
In fact, when the same current is supplied to the bonding wires, if there is a single bonding wire in tact, all of the current flows in this single bonding wire, thus generating dissipation of power which is four times greater than in the case of correct bonding, in which the current is distributed substantially equally between the two bonding wires.
As a result, in the case of incorrect bonding, there is greater heating of the bonding wire, a greater increase in its resistance, and thus a higher voltage drop, and therefore, when the development over a period of time of the voltage drop is compared with the corresponding development over a period of time for the same bonding in a part which is known to be intact, it is possible to detect any absence of a bonding wire.
However, practical implementation of this solution has the disadvantage that it requires firstly generation and control of relatively high currents, and secondly, use of highly sensitive measuring devices, since, in view of the resistance values of the gold wires concerned, the devices must be able to distinguish variations of voltage that are always very small, of approximately a few mV.
A solution proposed in order to eliminate the disadvantages inherent in the known art is described in patent application EP-74930, in the name of the applicant.
This solution involves doubling not only of the pad, but also of a section of the line which connects the pair of doubled pads electrically to a terminal of the electrical component, thus obtaining a pair of current paths arranged in parallel with one another, between the doubled pads and the terminal of the electronic component to which these pads are connected.
If a current is now supplied along the connection line, there is a voltage drop along the doubled sections of the connection line that are connected to intact bonding wires, but not along the doubled sections of the connection lines that are connected to bonding wires that are broken or not bonded.
By then measuring by means of comparators whether there is a difference of potential between the doubled contact pads, it is possible to determine any interruption of one or more bonding wires.
However, practical implementation of this solution on silicon has the disadvantage that it requires firstly the provision of precise rules of layout, to be followed in order to form the bonding, and secondly, additional circuitry, including the comparators, for recognition of the intactness of the bonding.
The embodiments of the present invention provide an electronic device with multiple-wire bonding, a manufacturing method thereof, and a method for checking the intactness of the bonding, which make it possible to eliminate the above-described disadvantages of the known art, by minimizing the complexities of the circuitry, both from the point of view of the circuits that are necessary in order to recognize the intactness of the bonding, and from the point of view of driving of these circuits, and, at the same time, by making it possible to provide a simple and reliable final test phase.
According to the present invention, an electronic device is provided that includes a first and at least a second external pin; a first, a second, and at least a third pad; a first bonding wire connecting the first pad to the first external pin; a second bonding wire connecting the second pad to the first external pin; a third bonding wire connecting the third pad to the third external pin; and a first and at least a second component having respectively a first and a second electrical impedance, and having first terminals connected respectively to the first and second pad and second terminals connected to a first node, the first node also being connected to the third pad.
According to the present invention, a method for manufacturing an electronic device is also provided that includes providing a first and at least a second external pin; providing a first, a second, and at least a third pad; bonding a first bonding wire between the first pad and the first external pin, a second bonding wire between the second pad and the first external pin, and a third bonding wire between the third pad and the second external pin; providing a first and at least a second component having respectively a first and a second electrical impedance; connecting first terminals of the first and second component respectively to the first and second pad, and second terminals of the first and second component to a first node; and connecting the third pad to the first node.
Finally, according to the present invention, a method for checking intactness of bonding wires of an electronic device is provided, the device having a first and at least a second external pin; a first, a second, and at least a third pad; a first bonding wire connecting the first pad to the first external pin; a second bonding wire connecting the second pad to the first external pin; a third bonding wire connecting the third pad to the second external pin; and a first and at least a second component having respectively a first and a second electrical impedance, and having first terminals connected respectively to the first and to the second pads, and second terminals connected to a first node, the first node also being connected to the third pad, the method including carrying out at least one impedance measurement between the first and the second external pins. Ideally, the method includes comparing the measurement to a predetermined value to determine the intactness of the bonding wires.